csc 480 software engineering lecture 17 nov 4, 2002
DESCRIPTION
Objectives To explain the advantages and disadvantages of distributed systems architectures To describe different approaches to the development of client-server systems To explain the differences between client-server and distributed object architectures To describe object request brokers and the principles underlying the CORBA standardsTRANSCRIPT
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CSC 480 Software Engineering
Lecture 17Nov 4, 2002
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Topics covered Multiprocessor architectures Client-server architectures Distributed object architectures CORBA
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Objectives To explain the advantages and disadvantages of
distributed systems architectures To describe different approaches to the
development of client-server systems To explain the differences between client-server
and distributed object architectures To describe object request brokers and the
principles underlying the CORBA standards
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Distributed Systems
Virtually all large computer-based systems are now distributed systems
Information processing is distributed over several computers rather than confined to a single machine
Distributed software engineering is now very important
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System Types
Personal systems that are not distributed and that are designed to run on a personal computer or workstation.
Embedded systems that run on a single processor or on an integrated group of processors.
Distributed systems where the system software runs on a loosely integrated group of cooperating processors linked by a network.
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Distributed System – characteristics
Resource sharing Openness Concurrency Scalability Fault tolerance Transparency
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Distributed System – disadvantages
Complexity Security Manageability Unpredictability
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Design issue Description Resource identification
The resources in a distributed system are spread across different computers and a naming scheme has to be devised so that users can discover and refer to the resources that they need.
Communications For most distributed systems, Internet is the most effective way for the computers to communicate. However, where performance and reliability are critical, alternative approaches to communications may be used.
Quality of service
The quality of service offered by a system reflects its performance, availability and reliability.
Software architectures
The software architecture describes how the application functionality is distributed over a number of logical components and how these components are distributed across processors. Choosing the right architecture for an application is essential to achieve the desired quality of service.
Distributed System – design issues
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Distributed Systems – architectures
Client-server architectures Distributed services which are called on by clients.
Servers that provide services are treated differently from clients that use services
Distributed object architectures No distinction between clients and servers. Any object
on the system may provide and use services from other objects
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Middleware Software that manages and supports the
different components of a distributed system. In essence, it sits in the middle of the system
Middleware is usually off-the-shelf rather than specially written software
Examples Transaction processing monitors Data converters Communication controllers
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Multiprocessor Architectures
Simplest distributed system model System composed of multiple processes which
may (but need not) execute on different processors Architectural model of many large real-time
systems Distribution of process to processor may be pre-
ordered or may be under the control of a dispatcher
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A multiprocessor traffic control system
Traffic lights
Lightcontrolprocess
Traffic light controlprocessor
Traffic flowprocessor
Operator consolesTraffic flow sensors
and cameras
Sensorprocessor
Sensorcontrolprocess
Displayprocess
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Client-server Architectures The application is modelled as a set of services
that are provided by servers and a set of clients that use these services
Clients know of servers but servers need not know of clients
Clients and servers are logical processes (not physical machines)
The mapping of processors to processes is not necessarily 1 : 1
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A Client-Server System
s1
s2 s3
s4c1
c2 c3 c4
c5
c6c7 c8
c9
c10
c11c12
Client process
Server process
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Computers in a C/S Network
Network
SC1SC2
CC1 CC2 CC3
CC5 CC6CC4
Servercomputer
Clientcomputer
s1, s2 s3, s4
c5, c6, c7
c1 c2 c3, c4
c8, c9 c10, c11, c12
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Layered Application Architecture
Presentation layer Concerned with presenting the results of a computation
to system users and with collecting user inputs Application processing layer
Concerned with providing application specific functionality e.g., in a banking system, banking functions such as open account, close account, etc.
Data management layer Concerned with managing the system databases
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Application LayersPresentation layer
Application processinglayer
Data managementlayer
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Thin and Fat Clients Thin-client model
In a thin-client model, all of the application processing and data management is carried out on the server. The client is simply responsible for running the presentation software.
Fat-client model In this model, the server is only responsible for data
management. The software on the client implements the application logic and the interactions with the system user.
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Thin and Fat clients
Thin-clientmodel
Fat-clientmodel Client
Client
ServerData management
Applicationprocessing
Presentation
Server
Datamanagement
PresentationApplication processing
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Thin Client Model
Used when legacy systems are migrated to client server architectures. The legacy system acts as a server in its own right
with a graphical interface implemented on a client A major disadvantage is that it places a heavy
processing load on both the server and the network
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Fat Client Model
More processing is delegated to the client as the application processing is locally executed
Most suitable for new C/S systems where the capabilities of the client system are known in advance
More complex than a thin client model especially for management. New versions of the application have to be installed on all clients
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A Client-Server ATM System
Account server
Customeraccountdatabase
Tele-processing
monitor
ATM
ATM
ATM
ATM
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Three-tier architectures In a three-tier architecture, each of the
application architecture layers may execute on a separate processor
Allows for better performance than a thin-client approach and is simpler to manage than a fat-client approach
A more scalable architecture - as demands increase, extra servers can be added
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A Three-Tier C/S architecture
Client
Server
Datamanagement
PresentationServer
Applicationprocessing
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An Internet Banking System
Database server
Customeraccountdatabase
Web server
Client
Client
Client
Client
Account serviceprovision
SQLSQL query
HTTP interaction
Account serviceprovision
Application Server
HTML filetransfer
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Use of C/S ArchitecturesArchitecture ApplicationsTwo-tier C/Sarchitecture withthin clients
Legacy system applications where separating applicationprocessing and data management is impracticalComputationally-intensive applications such as compilers withlittle or no data managementData-intensive applications (browsing and querying) with littleor no application processing.
Two-tier C/Sarchitecture withfat clients
Applications where application processing is provided byCOTS (e.g. Microsoft Excel) on the clientApplications where computationally-intensive processing ofdata (e.g. data visualisation) is required.Applications with relatively stable end-user functionality usedin an environment with well-established system management
Three-tier ormulti-tier C/Sarchitecture
Large scale applications with hundreds or thousands of clientsApplications where both the data and the application arevolatile.Applications where data from multiple sources are integrated